1. Technical Field
The present invention relates to a position sensor and to parts therefor. The invention has particular, although not exclusive, relevance to stylus input computer and communication devices, particularly small, low-cost devices, such as personal digital assistants (PDAS), mobile telephones, web browsers and combinations of these. The invention has particular relevance where those computer and communication devices are battery-powered.
2. Related Art
Several pen or stylus sensing systems for computer input exist. For example, U.S. Pat. No. 4,878,553 and the applicants' earlier International Application No. WO 00/33244 describe inductive stylus position sensing systems which allow for handwriting input, menu selection and other similar applications.
In WO 00/33244, processing electronics in the computer device generates an AC current, which is fed to an excitation coil in a sensor board of the device. This current generates an AC magnetic field that can couple with a coil in the stylus. A capacitor is also provided in the stylus connected in parallel with the coil to form a resonator. The magnetic field from the sensor board forces the resonator in the stylus to resonate. When the AC current is removed from the excitation coil, the resonator continues to resonate, with the amplitude of the oscillation decaying exponentially with time. This generates similar decaying EMFs in sensor coils on the sensor board, which are processed by processing electronics to provide a position indication of the stylus relative to the computer device.
In order to mimic the action of a conventional pen, the system described in WO 00/33244 also detects when the electronic stylus is pressed against a writing surface of the device by arranging the stylus so that the stylus resonator's frequency varies as a function of pressure applied to the nib of the stylus. The processing electronics in the device can then detect the resonator frequency in order to infer the nib pressure. In most PDA and similar applications, only a “clicked” or “unclicked” (i.e., stylus touching the writing surface or not touching the writing surface, respectively) indication of nib pressure is required.
In WO 00/33244, the position processor normally operates with a fixed excitation frequency which it uses to excite the resonator in the stylus. The position processor then detects the electrical phase of the return signal in order to infer the pen resonator frequency. The electronic stylus described in this earlier International application is designed to provide a well-defined difference between the clicked and unclicked frequency (hereinafter the click-shift frequency). However, the absolute value of those frequencies is variable between styluses and the amount of variability may typically be greater than the click-shift frequency. As a result, a single measurement of the resonant frequency of the stylus may be insufficient to determine whether it is clicked or unclicked.
One possible solution to determine click status is to perform a special tuning step before the stylus can be used normally, such as requiring the user to put the stylus into a known state (for example, in the clicked state by touching the stylus against the writing surface) and to store the resonant frequency of the stylus in this state. In subsequent normal operation, the stylus state is reported as clicked if the resonator frequency is measured close to the previously stored value and not clicked if the difference is greater than a predetermined threshold. However, such a tuning technique has the drawback that it requires cooperation from the user. Ideally, if tuning is to be performed, it should be done in a manner that is transparent to the user.
Another solution to this problem would be for the position processor to continuously track the position of the stylus in order to predict when the stylus is in a particular state, at which point its frequency is measured and used as a reference. However, such prediction is difficult and for low-powered devices (such as hand-held battery-powered devices) requires excessive power to be drawn from the battery if the stylus is to be tracked continuously. Further, without continuous tracking, it is difficult to detect the condition where a user swaps between two styluses with different frequencies (which might occur if several styluses are provided, each associated with a different function, such as writing and erasing).
Another problem associated with the stylus design described in WO 00/33244 is that the resonant frequency of the stylus can reduce significantly if the stylus is rested flat on the writing surface, due to magnetic screening used behind the sensor board of the hand-held device. In this case, the processor may erroneously report that the stylus has been clicked.
Some of these problems would be overcome in the stylus described in WO 00/33244 by simply increasing the click-shift frequency. However, with the design of stylus described in WO 00/33244, this would require significant movement of the nib of the stylus between the clicked and unclicked states, which would feel unacceptably large for users.